The female of Ergasilus sittangensis exhibits an oblong cephalothorax, with the first pedigerous somite remaining distinct from it. On the dorsal surface of the cephalic shield, there is an inverse T-shaped marking. The second to fourth pedigerous somites are free and taper posteriorly. The antennules consist of six segments, while the antennae have four segments, featuring a short coxabasis armed with inner setae, a second segment nearly twice as long as the coxabasis, a curved third segment with minute spines, and a terminal segment modified into a strongly curved claw. The mandible comprises anterior, middle, and posterior blades. The maxillule is lobate with setae, a middle process, and spinules, while the maxilla has rows of sharp teeth anteriorly. Swimming legs I to IV have three-segmented rami, except for the two-segmented exopod in the fourth leg. The fifth leg is two-segmented. The genital double-somite is wider than long and adorned with five rows of ventral spinules. The first free abdominal somite is slightly longer than the second one, with urosomites separated by a zone of folded cuticle. The somite of Ergasilus sittangensis is smaller than the second abdominal somite, and the posterior margin of each abdominal somite is ornamented with rows of spinules. The caudal rami are notably longer than the anal somite, and the egg sacs are long and cylindrical. The coloration of this species is white creamy, and it typically measures an average body length of 0.78 mm and a width of 0.24 mm.